Premixing effects on NOx scaling in high-pressure, lean methane-air axial stage combustion

Reducing pollutant emissions continues to be a primary concern for the development and operation of power generation systems to minimize environmental impacts. Operating combustors under fuel-lean conditions with enhanced premixing is a proven strategy for reducing NO_x emissions, but a deeper understanding of NO_x formation mechanisms across a range of engine-relevant conditions is vital. Recent studies have shown that the dominant NO_x formation mechanism shifts from prompt to thermal when increasing pressure from atmospheric to high pressure conditions, emphasizing the need for system relevant data. This study presents emissions measurements of a high-pressure, lean axially staged combustion experiment designed to quantify changes in NO_x production relative to equivalence ratio and fuel-air premixing. For each test case, constant vitiated crossflow conditions are supplied to the secondary combustion zone which consists of a lean methane-air reacting jet in crossflow. Results show that NO_x emissions increase with increased jet equivalence ratio and with reduced fuel-air premixing, as less premixed jets create locally rich regions that promote NO_x-forming hotspots. To collapse the measured NO_x trends, two scaling strategies are applied. First, flame and post-flame NO_x contributions are quantified through detailed chemical kinetics across a spread of equivalence ratio. Second, the effects of premixing are captured using mixture fraction distributions in the axial jet injector, defined by CFD simulations of the injector mixing at each tested equivalence ratio. When experimental data are first scaled using only the individual NO_x production rates, results converge well for highly premixed conditions, effectively collapsing equivalence ratio effects, but show scatter at lower premixing levels. Incorporating the CFD-derived mixture fraction distributions further collapses the data, eliminating observable trends with premixing and indicating successful normalization of mixing effects. This approach resulted in a mean scaled emissions value of 0.34 with a standard deviation of about 25 %. The result of the current study is an effective collapse of experimental NO_x emissions across both global (equivalence ratio) and local (jet premixing) fuel-air variations in lean, premixed, axially staged flames - an operating regime not previously characterized in this manner.